Without limiting the scope of the invention, its background is described in connection with uses of Bacillus thuringiensis toxins as cotton insect biocidal agents, as an example. Cotton insect pests reduced yields by almost 10% across the US in 1998. Insect damage reduced the overall cotton yield by more than 1.7 million bales and produced a financial loss of about $1.224 billion. One group in particular, the bollworm/budworm complex was the most damaging causing a 2.7% loss. The pink bollworm, Pectinophora gossypiella Saunders (“PBW”), is a lepidopteran insect that causes severe damage to cotton and is the most destructive pest of cotton worldwide.
Bacillus thuringiensis is a gram positive, sporeforming bacterium that forms a parasporal crystal which contains insecticidal toxins (Bulla et al., Crit. Rev. Microbiol. (1980) 8: 147–204; Höfte and Whiteley, Microbiol. Rev. (1989) 53: 242. The effect of the toxin is mediated through binding to specific receptors on the apical brush border of the midgut microvillae (BBMV) of susceptible insects.
Biological control of cotton pests using B. thuringiensis formulations and transgenic plants has been in use for a number of years and is growing rapidly. Recently, transgenic cotton plants carrying the toxin genes of BT have been developed and sold commercially. Such transgenic plants have a high degree of resistance to the pink bollworm (Schnepf et al., Microbiol. Mol. Biol. Rev. (1998) 62: 775). However, the introduction of any new insecticide into a pest management program immediately initiates a selection process for individuals that are resistant to the pesticide. As the use of transgenic crops expressing BT toxin increases, insect resistance is expected to become more widespread. Increased tolerance for BT toxins in several species of insects has been reported by several investigators while laboratory selection experiments have shown that the use of BT toxin formulations and transgenic plants can provoke the development of resistance in the pink bollworm (Bartlett, et al., Beltwide Cotton Conference (1995) 2: 766).
Concerns that BT toxin formulations or transgenic plants expressing the toxin genes may evoke emergence of either resistant or tolerant strains of insects has made the search for a better understanding of the interaction between the BT toxin proteins and their respective insect receptors a matter of considerable economic importance.
In U.S. Pat. No. 5,693,491, the present inventors disclosed the purification and cDNA cloning of a B. thuringiensis toxin receptor BT-R1 from larvae of the tobacco hornworm Manduca sexta (M. Sexta). Recently, two BT toxin receptors have been identified, purified and cloned from the silkworm, Bombyx mori (Nagamatsu et al., Biosci. Biotechnol. Biochem. (1998) 62: 727).
Heretofore in this field, there has been no structural information concerning the structure and function of BT toxin receptor of the major cotton insect pest, P. gossypiella. Furthermore, to the inventors' knowledge, the minimum binding fragment encoding a consensus binding domain for BT toxin on the BT receptor has not yet been identified. Isolation of the minimum binding fragment could permit cloning and structural characterization of important yet uncharacterized BT toxin receptors from other insects of worldwide economic importance such as P. gossypiella. 